To comply with the current National Electrical Code, conduit seals must prevent the passage of gases, vapors, or flames from one portion of an electrical installation to another when used in areas where flammable or explosive gases and vapors may be present. For this reason, hermetically sealed conduits have been developed. One method and apparatus for accomplishing this goal can be found in U.S. Pat. No. 4,640,978 issued to the Minnesota Mining and Manufacturing Company. This patent relates to a method of sealing electrical fittings by foaming in situ a removable, flame retardant, solvent resistant, substantially closed cell isocyanurate resin. The patent provides that once the conduit and conduit fitting are installed and wires threaded through the conduit, a nonflammable packing material is inserted into the fitting to prevent the flow of the resin beyond the area that needs to be sealed. The resin compound is then mixed and poured into the fitting through an opening, which is closed with a seal plug once the area to be sealed is filled with the resin compound. A drawback of this invention is that the composition contains water, which can cause the composition to swell at low temperatures and fracture the seal casing. Another drawback of this design is that the nonflammable packing material used to section off the area to be sealed is extremely difficult to insert in a manner necessary to prevent the migration of the composition into the conduit and out of the seal body.
At least one prior art conduit seal has eliminated the use of packing material to section off the area to be sealed. This device is disclosed in U.S. Pat. No. 2,247,671 issued to Tepel et al. In this device, the conduit seal is mounted on the end of the conduit and consists of three separate body-forming elements, an inner casing or coupling member, an extension or intermediate casing member, and an outer end or ring bushing member. The inner casing member is formed with an interior thread adapted to be screwed onto the threaded end of the conduit. Adjacent to the threaded portion of the inner casing and opposite the end of the conduit is an annular shoulder or abutment. Between the end of the conduit and the abutment is a soft rubber diaphragm mounted between two relatively thin annular sheet metal rings. The rubber diaphragm is formed with an opening of a diameter considerably smaller than the outside diameter of the sheath of the cable. This is so that when the cable is inserted in the opening, the rubber snugly embraces the cable in a substantially tight manner. Thus, the rubber diaphragm in this device replaces the use of the packing material. However, a drawback of this device is that it is designed as an end seal, primarily for lead sheathed cables. It is not designed to join two lengths of conduit and provide a seal that will deter the flow of gases, vapors, and flames. Also, it is not easily connectable to the conduit.